WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I do not think that you can buy vacuüm tube related stuff in Amsterdam.
Behind the central station you can get on a fast hydrofoil boat to IJmuiden.
There is BACO, kind of surplus store. Use google